The root mean square (RMS) error of the processed surface elevation values were derived from survey line crossings. Line crossings did not always capture the same point on the seafloor (because of GPS accuracy, navigation, currents, and timing of data recording). Therefore, variability in elevation values at crossings is likely in part derived from variability in the bathymetry of the seafloor where line crossings spanned 0.5 to 2 m apart in the horizontal.
Because there can be significant natural variability in depth/elevation across 0.5 to 2.0 m of the seafloor, the vertical error of the survey was determined from analysis of three classes of elevation values at line crossings:
1) values within 2 m of each other from all areas of Westcott Bay including vegetated, rocky and smooth areas (RMS=9.8 cm, n=66)
2) values within 2 m of each other from the smooth area between Bell Point and the head of Westcott Bay (RMS=3.6 cm, n=22)
3) values within 0.5 m of each other within the smooth area between Bell Point and the head of Westcott Bay (RMS=2.6 cm, n=6)
The overall error for the entire survey is therefore 9.8 cm, although particular areas, especially characterized by low relief likely have much smaller errors.
B) Sediment Samples for Grain Size Analyses
Triplicate samples were collected at sites WBN and BP for error analyses.
Analyses of triplicate samples collected at stations WBN and BP show that inter-station variability around the mean grain size ranged ±0.054 mm. The results were then gridded across the bathymetric surface with a nearneighbor gridding algorithm averaging between the three nearest points.
Video of the seafloor was collected along several principal transects in the western and central portions of Westcott Bay to ground-truth the sonar data.
C) Nearshore Currents
Select transects at the mouth of Westcott Bay, Bell Point and immediately east of the Westcott Sea Farm were repeated ~2-3 times under flooding and ebbing tides.
Sixty-nine (69) kilometers of acoustic bathymetry/substrate data were collected with a dual-frequency (200 and 430 kHz) Biosonics system along 142 transects in Westcott and Garrison Bays between 5/31/07 and 6/2/07. The Biosonics sonar generates a 6-degree cone of sound, which translates into a footprint on the seafloor ranging 0.2 to 0.8 m for the water depths surveyed (2 - 8 m). Ship speed generally ranged 4.0-4.5 kts and data were merged with GPS positions at 1 Hz. Resulting data therefore represent 0.2-0.8 m pixels on the seafloor spaced approximately 1-2 m apart along track lines. Track lines were spaced 25 m apart. Twenty (20) km of topographic elevation data were also collected across the upper intertidal region by walking the shoreface with a portable Trimble 4700 RTK-DGPS receiver. This receiver utilized a Trimble Zepher Antenna and Pacific Crest radio receiver to obtain real time position corrections from a base station operating a Trimble 4400 receiver and L1/L2 antenna with a Pacific Crest 35 Watt radio transmitter. Elevation data over emergent beaches and tide flats collected by walking are 2-3 times denser as a result of survey speeds ranging 0.5-1.0 kts.
B) Sediment Samples for Grain Size Analyses
Sediment samples were collected using a van veen grab sampler at 71 stations on a 200-250 m grid to characterize grain size distribution throughout Westcott Bay and sources of fine material for suspension and transport.
The shallow stratigraphy was well-preserved in the sediment grabs so that a 1-3 cm thick aerated surface layer could be observed at most stations dominated by fines.
In the field, a sub-sample of the uppermost 1-3 cm of each grab was collected and placed into storage bags, labeled, logged, and frozen. Several 10-cm push cores were also collected.
In the laboratory, sediment samples were split for grain size and carbon analyses and later archived at the Western Coastal and Marine Geology Team sediment laboratory in Menlo Park, CA. Samples were disaggregated and sieved through 2000 and 62 micron screens to separate gravel, sands and mud after the organic component was removed by treatment with hydrogen peroxide. Sediment grain size of sands was determined using the settling methods and principles of Stokes Law on the USGS 3-m long settling tubes. The fine fraction (silts and clays) were analyzed on a Coulter 230 Laser Particle Analyzer. Three samples had less than 3% intermediate fraction and were analyzed on the Coulter. Single sample runs were made on the tubes, while results from the laser particle analyzer are averages of triplicates with standard deviations around the mean ranging 18 to 24 microns due to instrument errors.
C) Nearshore Currents
Current velocity, direction and backscatter amplitude were collected along 65 transects in Westcott Bay with a 600 kHz RDI ADCP (Acoustic Doppler Current Profiler) and recorded with WinRiver software. ADCP were recorded with GPS position at 1 hz with a ship speed of 2.0-2.5 kts, so raw ADCP data cover a lateral distance of 1.00 to 1.25 m.
The combined bathymetric and topographic data were merged to create a digital elevation model representing the surface topography of Westcott Bay. This surface is referenced to the WGS 84 datum in the UTM Zone 10 North projection with a horizontal accuracy of 2.3 cm. Elevations are referenced to NAVD88 with an estimated vertical accuracy ranging 2.6-9.8 cm. This includes error from the RTK-DGPS and Biosonics sonar, and errors introduced in data processing.
Results from the tubes and laser particle analyzer were merged using standard USGS methods found in the USGS particle analysis program pcSedSize.
The raw ADCP data were vertically binned at 0.25 m and include a blanking distance of 0.25 m (no data in uppermost 0.25 m below the transducer). The RDI ADCP is accurate to within 0.25% of boat + water velocity, resulting in an error for our velocity data of 0.25 to 0.45 cm/s based on our survey speed of 2 kts and measured velocities ranging 0 to 1.5 m/s.
Sample of digital surface elevation data XYZ ASCII file, bathy/line95.xyz: X Y Z 487989.879521 5381998.198351 -3.31 487990.116289 5381998.892258 -3.34 487989.943243 5382001.029467 -3.40 Sample of Acoustic Doppler Current Profiler XY ASCII file, ADCP/RAW/WESC053107_010p.xy: X Y 487869.695129 5382286.342641 487870.234235 5382287.490111 487871.174033 5382288.284718 Sample of sediment sampling log: Station Local_Time UTC Lon Lat Depth_m Comment 1 6/1/07 20:44 6/2/07 3:44 -123.1430877 48.60642247 2.286 - 2 6/1/07 20:35 6/2/07 3:35 -123.1416559 48.60596625 2.1336 18.6 C 3 6/1/07 20:28 6/2/07 3:28 -123.1396819 48.60476647 2.4384 22.4 C
Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.